COUNTERPOINT NPA

Commentary

A monaural power amplifier with a completely non-feedback configuration.

NPA stands for Natural Progression Monaural Amplifier (NPA).

The NPA combines a 6DJ8 vacuum tube input stage and an insulated gate bipolar transistor (IGBT) output stage. The IGBT sends the sound unique to vacuum tube amplification to the speaker with a high current output of 250A.
By designing a circuit that effectively utilizes the IGBT, which has the characteristics of both a BJT (bipolar junction transistor) with low internal loss and a MOS FET with high current gain, and by simplifying the drive circuit using the IGBT, we have achieved a high current power amplifier that eliminates the sound quality inherent in the transistor.

The NPA uses a unique gain stage design to improve linearity and signal purity.
In this method, the cathode and cathode of two triodes are connected, one cathode is driven through the other cathode, the cathode of the triodes is directly coupled to a constant current load, eliminating the loss of high-frequency detail and signal dynamics, and the distortion factor is reduced by adopting a constant current load in the plate.

The NPA employs an open-loop amplifier circuit to eliminate transient IM distortion and realize a perfectly balanced symmetrical input.
Although NFB is used in general amplifiers, it is effective in improving characteristics, but in principle it has an extremely large loop, so it is not suitable for operations that handle music signals. In NPA, since high-frequency distortion is suppressed by the configuration of double couple 3-pole tubes, there is no need for an overall feedback loop to reduce the distortion factor. Since there is no feedback loop in the input stage, mutual interference between the power amplifier and the preamplifier can be reduced, making it easier to match devices.
In addition, the vacuum tube amplifier has the advantage of being less affected by the reverse feedback signal because of its high input impedance. In addition, the open-loop first stage design provides a completely symmetrical input stage so that the preamplifier can perform push-pull operation at a matched load.
In the NPA, the DC servo is omitted, and the DC loop is incorporated only in the output stage. This reduces interference between the servo and the signal while taking advantage of DC servo compensation.

Both XLR and RCA connectors are used as input jacks.
Since the NPA is a fully balanced input circuit, it is generally recommended to use the XLR connector. The RCA connector has a positive terminal for positive-phase input and a negative terminal for connection to a preamplifier that inverts the signal polarity. It can also be used with preamplifiers that have separate positive and negative outputs.

Since two pairs of output terminals are connected in parallel, both pairs can be used for a bi-wiring connection.

A mute switch is mounted on the rear panel.
This switch can be used to connect or disconnect speakers smoothly without worrying about shock noise when connecting or disconnecting cables.

The chassis is assembled from 30 aluminum and copper plated steel parts, all of which are manufactured and processed by Counterpoint's Precision Sheet Metal Processing Department in Carlsbad, CA.
In addition, the side of NPA is reinforced with wood panels.

The power transformer is a 2000 VA transformer designed and manufactured by our Precision Magnetic Division.
The transformer is physically isolated from the chassis for quiet operation.

It uses parts such as German metal film 1% resistance and TRT film capacitor.

An output fuse is used as a protective function, and a mute circuit is installed that continuously monitors the output voltage of the servo and operates when the voltage exceeds the specified limit due to input overdrive or other reasons.

Model Rating

Type	Tube type monaural power amplifier
Rated output	300W (4 Ω) 150W (8 Ω)
Input impedance	100k Ω (To ground, all inputs)
Input sensitivity	1.15 Vrms at maximum output
Gain	29.7dB
Frequency characteristic	3 Hz to 100 kHz -3dB
Signal-to-noise ratio	90 dB or more (1W, IHF WTD)
Slew rate	25 V / μ s
Current supply capability	250A(peak to peak)
Power consumption	150W (No Signal)
External dimensions	Width 483x Height 172x Depth 470 mm
Weight	32kg